STP856

    Some Observations on J-R Curves

    Published: Jan 1985


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    Abstract

    The effect of specimen size and geometry on the onset of ductile tearing (Ji, δi) and the initial slope of J(dJ/da) and crack opening displacement dδ/da resistance curves has been investigated for a carbon-manganese steel. The results indicate that in plane sided specimens a thickness in excess of that specified in ASTM Test for JIc, a Measure of Fracture Toughness (E 813) is required to achieve a lower bound, or plane strain, Ji value. However, the resultant decrease in Ji is relatively small. Side-grooving was found to be very effective in promoting plane strain conditions in subsized specimens. The initial tearing resistance was found to be markedly influenced by specimen thickness and degree of side-grooving, and the results indicate that achieving plane strain conditions in the proximity of the crack tip does not ensure a unique J-R curve in side-grooved specimens.

    JIc values obtained using ASTM E 813 were found to overestimate the “true” Ji values for a carbon-managanese steel and a pressure vessel steel. This arises from the linear representation of the R curve and the assumed blunting line behavior A power law equation was found to give a statistically better fit to the J-Δa data, particularly in high toughness materials. The need for an engineering or working definition of ductile crack initiation for structural assessment has been acknowledged, and an alternative to the ASTM E 813 procedure suggested, based on the J value at 0.2 mm of crack extension.

    Keywords:

    elastic-plastic properties, ductility, steels, elastic-plastic fracture, J, integral fracture mechanics, crack opening displacement, stable crack growth, ductile crack initiation


    Author Information:

    Gibson, GP
    Higher scientific officer and senior scientific officer, AERE Harwell, Oxfordshire,

    Druce, SG
    Higher scientific officer and senior scientific officer, AERE Harwell, Oxfordshire,


    Paper ID: STP34524S

    Committee/Subcommittee: E08.08

    DOI: 10.1520/STP34524S


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